It is well known for flight actuator to include two mechanical load paths, one primary and the other secondary, with the latter intended to take up the load when the primary path has failed. In a typical prior art device, as shown in FIG. 1, when operating on the primary load path the loads are transmitted through a hollow ball or roller screw. The hollow screw houses a safety rod, also called a failsafe bar or tie bar, which is connected to the screw with a small amount of play. During normal operation of the screw, when the primary load path is working correctly, the secondary load path formed by this tie bar carries no load since there is no contact due to the small amount of play. However, in the event of a failure of the screw in the primary load path then the tie bar performs its failsafe function and ensures continuity of the transmission of loads by the actuator.
With reference to FIG. 1 a typical known flight control actuator of the trimmable horizontal stabiliser actuator (THSA) type includes a primary load path with a hollow screw 32 connected at its upper end to the aircraft via a Cardan joint system 24 joining with first aircraft structural elements S1. The primary load path further includes a nut assembly 25 mounted on the screw 33, and the nut assembly 25 is connected to the stabiliser 22 of the aircraft, this connection being achieved for example by another Cardan joint system 26.
As mentioned above, the secondary load path is provided by means of a tie bar 29 which is within the screw 32. The tie bar 29 is terminated at its upper end by a male portion, in this case taking the form of a spherical head 27, which is mounted within a female portion on a fastening piece 28, in this case taking the form of a recess 210. The fastening piece 28 is connected to the structure of the aircraft via second aircraft structural elements S2. The known system may also include some means for preventing motion of the nut assembly 25 relative to the screw 32 and/or for fixing the stabiliser 22/Cardan joint 26 in place when the primary load path fails. Thus, the lower attachment, of which the nut assembly 25 is a part, could also include secondary load path elements used when the primary load path fails.
In the example known upper attachment shown in FIG. 1 it will be understood that should the screw 32 fail then load can be carried by the secondary load path since movement of the spherical head 27 is restricted by the upper and lower shoulders of the recess 210. Consequently, the stabiliser 22 can either be safely held in a single position (in the case where the lower attachment locks in place) or in some prior art arrangements it might be permitted to continue normal movement should the tie bar 29 be connected with the ballscrew 23 in such a way as to permit continued rotation of the screw 23 even after failure preventing it from carrying axial loads.
A flight actuator with the basic features discussed above can be found in the prior art, for example in U.S. Pat. No. 8,702,034 and in US 2013/105623.